Date:         Sat, 9 Sep 2006 19:48:02 -0700
Reply-To:     Pensioner <al_knoll@PACBELL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Pensioner <al_knoll@PACBELL.NET>
Subject:      Brake Stuff
In-Reply-To:  <200609082215.k88MFPjG023048@flpvm16.prodigy.net>
Content-Type: text/plain; charset="iso-8859-1"

In a nutshell the pads are heat insulators that maintain their friction
coefficient under high heat and pressure.  They serve to transfer the heat
of friction to the rotor without heating up the caliper.  The disk ideally
vents the accumulated heat to the air in which it moves by convection.  You
can calculate the amount of energy dissipated by knowing the deceleration
and the weight of the van.  Needless to say it is considerable.

The pad material is designed to operate over a range of temperatures.  The
coefficient of friction or 'braking power' of a pad varies with temperature
by design.  Mundane Vanagon pads need to be able to stop pretty well when
cold and sacrifices high heat braking power.  Racing pads need to stop when
very hot as the energy/heat dissipated is much greater. M(V2-V1)**2 is the
kinetic energy that must be dissipated where M is the mass of the van V2-V1
is the speed difference required. EG 70MPH to 30MPH.  The more severely you
brake the higher the temperature of the pad to disk interface produced.  At
some point the disk is hot enough to break down the pad chemically and
produce a glaze.  This surface is much slipperier than the unglazed surface
and has a much lower coeffiecient of friction and therefore requires much
more pressure to stop.

Brake fade need does not always imply glazing only that the ability of the
disk to accept more heat and hence cool the pad surface is decreased and
therefore also requires addtional pressure to achieve the same retarding
force.  When the brake surface cools again the brake functions as before.

Out of a nutshell the pecan is easier to eat.

Learning all this boring swill is left as an exercise to the reader.

"if you have time to brake you have time to miss the whole thing"
attributed to Jimmy Clark Lotus F1 Champion

To once again paraphrase Feynmann " It's just fizzix, it's always just
fizzix"

P